Method of making electric heating units



A rifl 29, 36 L. G. SPRAGUE I METHOD OF MAKING ELECTRIC HEATING UNITS Filed Dec. 1, 1966 INVENTOR. LLOYD G. SPRAGUE AGENT United States Patent 01 fice 3,440,720 Patented Apr. 29, 1969 3,440,720 METHOD OF MAKING ELECTRIC HEATING UNITS Lloyd G. Sprague, Bath, N.Y., assignor to Corning Glass Works, Corning, N.Y., a corporation of New York Filed Dec. 1, 1966, Ser. No. 598,307 Int. Cl. H01c 17/00, 3/00; B05b 7/20 US. Cl. 29-411 6 Claims The present invention relates to electric heating units and more particularly to electrical heating units for use in items, such as electric hot plates for example, each employing at least one electrical resistance heating element for temperature control of the respective item. Still more specifically the present invenion relates to improvements in electrical heating units of the type, for example, which have one or more electrical resistance heating elements sinuously or serpentinely wound on a card of dielectric material so that the element or elements extend back and forth across one side of the card in serpentine paths extending substantially parallel with and equally spaced from each other on said side of the card.

In a type of heating unit such as that mentioned, said serpentine paths abruptly and repeatedly turn 180 from one direcion to the opposite direction at selected regions on the card, such as adjacent the edges thereof. Such reversals of direction obviously also require, in said selected regions, 180 turns in the heating element or elements extending in said serpentine paths, that is, a looping back of the element or elements parallel with themselves or with each other. It is readily apparent, therefore that in said regions of turning, portions of the heating element or elements are substantially closer to each other than they are across the remainder of said dielectric card where the paths of the element or elements extend substantially parallel with each other. Said regions of turn, because of the closer proximity of portions of the heating element or elements to each other in such regions, can cause hot spots on said dielectric card which increase the possibility of burn out of the heating elements at such spots.

It is, accordingly, an object of the present invention to provide a method of eliminating said hot spots in heating units employing heating elements disposed in arrangements such as that described and in similar arrangements.

In accomplishing the above object of the invention, the electrical resistance of the heating element or elements used in the heating units is reduced onr lowered in selected portions thereof, thereby reducing the thermal output of such portions. The reduction or lowering of the electrical resistance in said selected portions is accomplished by flame spraying such portions with a powdered electrically conductive material, thereby coating said portions with additional conductive material and increasing the conductance thereof.

There is shown in United States Patent application Ser. No. 550,329, filed Apr. 5, 1966, now US. Patent No. 3,391,372, granted July 2, 1968, for Electric Heating Unit, and assigned to the same assignee as the present application, an electric heating unit employing two electrical resistance heater elements of a type similar to that described above. As an example only, the present invention will be discussed in detail in conjunction with the heating unit of said type.

The invention will best be understood with reference to the accompanying drawings wherein:

FIG. 1 is an elevational view of the front or one side of one type of electric heating unit which may be modified in accordance with the present invention,

FIG. 2 is an elevational view of a portion of the back or opposite side of the heating unit of FIG. 1, and

FIG. 3 is a view similar to FIG. 2 and illustrating one embodiment of the present invention.

Similar reference characters refer to similar parts in each of the figures of the drawings.

Referring to the drawings in detail, there is shown a heating unit 9 including a rectangular, relatively thin but rigid panel or card 10 of a dielectric material, such as mica for example. One edge 11 of card 10 is rabbeted to provide, in alternation therealong, a series of first and second spaced-apart notches such as 12 and 13 of first and second depths, respectively. The edge of card 10 opposite edge 11 is similarly rabbeted to provide therealong a series of notches such as 12a and 13a corresponding to notches 12 and 13, respectively, provided in edge 11. Such opposite edge is designated in the drawings by the reference character 11a.

A first electrical resistance heating element 14 is contiguously mounted against the front side 10a of card 10 by being wound on such card progressively and successively back and forth across such side of the card from near one unnotched edge 15 of the card toward the other unnotched edge 16 of the card and parallel with such edges. Such element extends through successive adjacent pairs of notches 12 rabbeted in edge 11 of the card and through successive adjacent pairs of notches 12a rabbeted in edge 11a of the card, and across those portions of back 1011 of the card which lie between each said pair of notches. Thus, said pairs of notches provide for reversal of element 14 in its back and forth path across the front of card 10. First and second ends of element 14 extend through orifices or holes 18 and 19, respectively, provided in card 10, and are secured to back 10b of the card in any suitable or convenient manner such as by a suitable adhesive or cement for example.

A second electrical resistance heating element 17 is mounted on front side 10a of card 10 in a manner similar to element 14 and extends parallel with element 14 progressively back and forth across the front of such card from adjacent the edge 15 of the card toward edge 16 thereof. Element 17 extends through successive adjacent pairs of notches 13 and 13a, and across those portions of back 10b of card 10 which lie between each respective pair of said notches. Thus, adjacent pairs of notches 13 and 13a provide for reversal of element 17 in its back and forth path across the front of card 10'. First and second ends of element 17 extend through orifices or holes 20 and 21, respectively, provided in card 10, and are secured to back 10b of the card in a manner similar to that in which said ends of elements 14 are so secured.

It is believed readily apparent that, by mounting elements 14 and 17 on card 10 in the manner described, such elements are arranged to cross each other adjacent he rabbeted edges 11 and 11a of the card without contact between the elements. The illustrated parallel relationship of elements 14 and 17 is thus attained, thereby providing for substantially uniform distribution of heat when heating unit 9, comprising card 10 and its associated elements 14 and 17, are employed, for example, for cooking purposes. That is to say, elements 14 and 17 may be electrically energized either singly or in combination and the heat provided by heating unit 9 is, thereby, substantially evenly distributed across the areal expanse of such unit.

It should be pointed out that heating elements 14 and 17 are illustrated in the drawings as ribbons of an electrical resistance material and, therefore, the bottom borders of notches 12, 12a, 13 and 13a are sloped so that such ribbons may be conveniently reversed in direction without undue twisting, creasing or folding of the ribbons. This is believed to be readily apparent. If the same width, as well as of different widths as illustrated in the drawings.

As will be readily apparent to those skilled in the art, in utilizing heating unit 9, the ends of elements 14 and of element 17 are connected across a suitable source or sources of electric current for energization of the elements. The electrical connections to said ends of elements 14 and 17 may be made by any of the well-known and convenient techniques, such as by soldering for example.

Referring now to the upper left hand part of FIG. 1 and to FIG. 2, it will be noted that in areas, such as 23 and 24, where elements 14 and 17, respectively, fold over or cross over themselves on opposite sides a and 10b of card 10, the drawings have been darkened to indicate such areas. Similarly, in those areas, such as 26, of said drawing where elements 14 and 17 cross over each other on opposite sides 10a and 10b of card 10, the drawings have been darkened to indicate those areas. Thus, the darkened areas of FIGS. 1 and 2 indicate regions of card 10 where elements 14 and 17 fold over or cross over themselves, or cross over each other. Alternatively, the darkened areas of FIGS. 1 and 2 may be said to indicate portions, such as 23 and 24, of elements 14 and 17, respectively, which are disposed opposite themselves on corresponding regions of opposite sides of card 10; and to also indicate portions, such as 26, of elements 14 and 17 which are disposed opposite each other on corresponding regions of opposite sides of said card.

As will be noted in viewing FIG. 1, heating elements 14 and 17 are substantially evenly spaced from each other throughout their serpentine paths except at edges 11 and 11a of card 10 where such elements extend through adjacent pairs of notches such as 12 and 12a, and 13 and 130, respectively, and cross back side 10b of the card in the regions of such side between said adjacent pairs of notches. Elements 14 and 17 provide, therefore, for substantially even heat distribution across side 10a of card 10 except in the regions of the card adjacent the portions of the heating elements shown darkened in FIGS. 1 and 2 of the drawings, that is, except in the regions of card 10 adjoining edges 11 and 11a of the card. In said regions of card 10, elements 14 and 17 are disposed closer to themselves and to each other than in any other regions of said card. That is to say, per unit area of said regions of card 10 adjoining edges 11 and 11a of the card, more of the lengths of elements 14 and 17 are disposed in such regions than in any of the remaining regions of the card and, therefore, hot spots occur in said regions. Such hot spots provide possible sources for burn out of elements 14 and/ or 17 due to the increased heat provided by such elements in the regions of the hot spots. Furthermore, in using a heating unit such as 9, the unit is usually disposed so that side 10: of card 10 and, thereby, the parallel lengths of the heating elements, such as 14 and 17, are in contact or in close thermal transfer relationship with one surface of a member to be heated, such as, for example, a plate member of a hot plate. Such disposition of the heater elements provides for relatively rapid transfer of heat, from said parallel lengths of the heating elements, to the member to be heated. However, in the areas where the heating elements cross over back 10b of card 10, such rapid transfer of heat from the heating elements does not occur, since, in such areas, the heating elements do not directly contact the member to be heated but contact the back of card 10. Thus, the problem of hot spots is further aggravated.

Referring now to FIG. 3 of the drawings, the improvement embodying the present invention will be discussed.

In order to eliminate the above discussed hot spots, it is necessary to decrease the thermal output of the heating elements, such as 14 and 17, in those portions of such elements, that is, in the regions of card 10, where such hot spots occur. This can be accomplished by reducing the electrical resistance of the heating elements in said portions thereof, and such reduction can be attained by increasing the conductance of the elements in such portions. As is believed obvious, it would be relatively time consuming and, therefore, expensive to attempt to increase the conductance of the heating elements in said portions'thereof prior to the winding of the elements on a paneljor card such as 10. Accordingly, the sections of elements 14 and 17 crossing the back of card 10 are, after such elements are wound on the card, each integrantly provided With an additional layer or coating of electrically conductive'material such as 27 (FIG. 3) to lower or reduce the electrical resistance of such sections of the elements. Such additional material is indicated in FIG. 3 by the shaded parts of elements 14 and 17.

In providingsaid additional material on the heating elements such as 14 and 17, card or panel 10, and a substantial part of such heating elements, are suitably masked in any convenient manner with only those portions of the heating elements crossing the back of the card left exposed. The conductive layer of material is integrantly deposited on the exposed portions of the heating elements by a flame spraying process. The electrically conductive material flame sprayed on said exposed portions of the heating elements may, for example, be Powdered Nickel XP1104 sold by Metco, Inc., whose address is Westbury N.Y., and such material may be deposited on said elements by a powder-type flame-spray gun such as described in United States Patent 2,961,335, issued Nov. 22, 1960, such a type of flame-spray gun being also sold by Metco, Inc. under the trade name of Thermospray powder gun. The techniques for depositing the layer of conductive material on the exposed portion of the elements may be similar, for example, to those discussed in United States Patent 3,254,970, issued June 7, 1966', to F. J. Dittrich et al., and assigned to the aforesaid Metco, Inc.

Flame spraying of the electrically conductive powdered material on the unmasked portions of elements 14 and 17 is continued until a layer or coating approximately 6 to 8 mils in thickness, for example, is deposited on the exposed or unmasked portions of the elements, that is, until a layer which is substantially equal to the thickness of the heating elemnets, such as 14 and 17, is integrally deposited on the unmasked portions of such elements. Such a layer lowers or reduces the electrical resistance of the coated portions of the heating elements to a degree sufficient to eliminate the aforementioned hot spots. The mask can then be removed from the card or panel, such as 10, and the heating elements, such as 14 and 17, and the heating unit, such as 9, can then be used for its intended purpose with a susbtantial increase in its dependability and in its durability.

The invention has been described herein specifically in conjunction with the elimination of hot spots caused by sinuously or serpentinely wound ribbon type heating elements. However, the invention is equally applicable to heating elements made in the form of wires and sinuously or serpentinely wound so as to be a possible source of hot spots. Furthermore, the invention can be applied to electrical resistance heating elements wound in configurations other than sinuous or serpentine configurations if such elements are wound so as to be a possible source of hot spots as herein described. It is also pointed out that powdered conductive materials and powder-type flame-spray guns other than those set forth herein can be used in practicing the invention.

Although there is herein shown and described only one specific embodiment and example of the practice of the present invention, it is to be understood that such is not intended to be in any way limiting but that changes and modifications may be made therein within the purview of the appended claims without departing from the spirit and scope thereof.

What is claimed is:

1. The method of increasing the dependability and durability of a heating unit spatially uniformly wound, except in minor regions thereof, with at least one linear electrical resistance heating element, such heating unit being spatially more closely wound in said minor regions; such method comprising, masking said unit so that the electrical resistance windings thereof are exposed only in said minor regions, depositing a layer of electrically conductive material on each of the exposed portions of said windings to lower the electrical resistance of such exposed portions and, thereby, the thermal output of such portions; and unmasking said unit and the masked portions of said windings; whereby, when electrical current is supplied to said unit for heating thereof, the possibility of hot spots occurring in said more closely wound regions of the heating unit is reduced or eliminated, and the possibility of burn out of said windings in said regions is accordingly decreased.

2. The method in accordance with claim 1 and in which said heating unit comprises a rectangular card of a dielectric material, said heating element comprises an electrical resistance ribbon in the form of windings extending back and fourth across one side of said card in equally spaced paths and reversing their directions in selected locations adjacent opposite edges of said card, and said minor regions comprise such selected locations.

3. The method in accordance with claim 2 and in which said conductive material comprises a powdered metal deposited on said exposed portions of said windings by flame spraying of the powdered metal.

4. In a heating unit including electrical resistance heating windings spatially uniformly Wound on such unit except in minor regions of the unit where said windings are spatially more closely wound; a method of increasing the durability of such heating unit by reducing or eliminating the possibility of occurrence of hot spots in said minor regions and, thereby, the possibility of burn out of said windings in one or more of said regions when electrical current is supplied to the heating unit for heating thereof; such method comprising, masking said heating unit and the major portions of said windings while leaving exposed only the portions of said windings located in said minor regions of the heating unit, and depositing an integrant coating of an electrically conductive material on the exposed portions of said windings to lower the electrical resistance of such portions of the windings and, thereby, the thermal output of such portions when electric current is thereafter supplied across said windings.

5. The method in accordance with claim 4 and in which said heating unit comprises a rectangular card of a dielectric material, said heating windings comprise at least one electrical resistance ribbon in the form of windings extending back and forth across one side of said card in equally spaced paths and reversing their directions in selected locations adjacent opposite edges of said card, and said minor regions comprise such selected locations.

6. The method in accordance with claim 5 and in which said conductive material comprises a powdered metal deposited on said exposed portions of said windings by flame spraying of the powdered metal.

References Cited UNITED STATES PATENTS 761,204 5/1904 Carpenter 29-611 1,392,944 10/1921 Jessup 338290 2,706,766 4/ 1955 Huffman 29-611 3,391,372 7/1968 McLean 338-290 US. Cl. X.R. 117105.2; 338290 

1. THE METHOD OF INCREASING THE DEPENDABILITY AND DURABILITY OF A HEATING UNIT SPATIALLY UNIFORMLY WOUND, EXCEPT IN MINOR REGIONS THEREOF, WITH AT LEAST ONE LINEAR ELECTRICAL RESISTANCE HEATING ELEMENT, SUCH HEATING UNIT BEING SPATIALLY MORE CLOSELY WOUND IN SAID MINOR REGIONS; SUCH METHOD COMPRISING, MASKING SAID UNIT SO THAT THE ELECTRICAL RESISTANCE WINDINGS THEREOF ARE EXPOSED ONLY IN SAID MINOR REGIONS, DEPOSITING A LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL ON EACH OF THE EXPOSED PORTIONS OF SAID WINDINGS TO LOWER THE ELECTRICAL RESISTANCE OF SUCH EXPOSED PORTIONS AND, THEREBY, THE THERMAL OUTPUT OF SUCH PORTIONS; AND UNMASKING SAID UNIT AND THE MASKED PORTIONS OF SAID WINDINGS; WHEREBY, WHEN ELECTRICAL CURRENT IS SUPPLIED TO SAID UNIT FOR HEATING THEREOF, THE POSSIBILITY OF HOT SPOTS OCCURRING IN SAID MORE CLOSELY WOUND REGIONS OF THE HEATING UNIT IS REDUCED OR ELIMINATED, AND THE POSSIBILITY OF BURN OUT OF SAID WINDINGS IN SAID REGIONS IS ACCORDINGLY DECREASED. 